In recent years, high pressure acid leach which uses sulfuric acid has been attracting attention as a hydrometallurgical process which is configured to recover nickel and cobalt from a nickel oxide ore containing iron as a main component and having a nickel content of 1% to 2% by mass. Unlike conventional common refining processes for nickel oxide ore, the high pressure acid leach does not include pyrometallurgical processing steps, such as a drying step and a roasting step, but includes consistent hydrometallurgical steps, and thus is advantageous in terms of energy and cost.
That is, the high pressure acid leach has a great advantage because, in a leaching step, the oxidation reduction potential and the temperature of a leachate in a pressure leaching reaction vessel are controlled to fix a main impurity, namely, iron, as a leach residue in the form of hematite (Fe2O3), whereby nickel and cobalt can be selectively leached out over iron.
Specifically, a nickel refining process using the high pressure acid leach includes, for example, the steps of: making a nickel oxide ore into slurry to prepare ore slurry (ore slurry preparation step); adding sulfuric acid to the ore slurry and leaching out nickel and cobalt under high temperature of 220 to 280° C. and high pressure by an autoclave to obtain leach slurry (leaching step); solid-liquid separating the leach slurry into a leach residue and a leachate containing nickel and cobalt (solid-liquid separation step); adjusting the pH of the leachate containing an impurity element together with nickel and cobalt to 3 to 4 to neutralize and separate the impurity element such as iron therefrom (neutralization step); and supplying hydrogen sulfide gas to the leachate obtained after the neutralization and separation to recover a nickel-cobalt mixed sulfide (sulfurization step), (for example, refer to Patent Document 1).
Here, in this nickel refining process, in the ore slurry preparation step, a nickel oxide ore is wet-grinded and sieved to obtain ore slurry which is adjusted to have a predetermined grain size. Then, in the leaching step, first, the ore slurry is made to pass through heaters provided in multiple stages, thereby being heated and pressurized step by step. Subsequently, in an autoclave, sulfuric acid is added under high pressure and temperature conditions produced by high-pressure steam, whereby nickel and cobalt and in addition some impurity elements are leached out into the liquid. Finally, the liquid is made to pass through flash tanks provided in multiple stages to reduce the temperature and the pressure of the liquid step by step.
As mentioned above, in the leaching step, using a pressurizer (autoclave) or the like which are made of very expensive materials, such as titanium, ceramics, and stainless steel, a leaching operation is performed usually under high temperature of 240 to 260° C. and high pressure. Therefore, in the high-pressure acid leach equipment to be used for the leaching step, also at the time of shutdown or startup for planned inspections and maintenance and the like, sufficient management based on predetermined standards is indispensable for security and equipment maintenance.
In the foregoing leaching step, when ore slurry is made to pass though the heaters provided in multiple stages to be heated and pressurized step by step, the ore slurry is pressurized in three stages (low pressure, middle pressure, high pressure), for example, and, in each of the stages, a high-pressure pump corresponding to the each stage is provided.
The high-pressure pump equipment provided in each of the stages is used under the strict conditions of temperature, pressure, and the like, and therefore, a periodic maintenance inspection needs to be conducted, and hence, a plurality of the pump-equipment circuits, for example, usually, two pump-equipment circuits are provided so that operations do not have to be stopped at the time of the maintenance inspection.
For example, in the two pump-equipment circuits, during normal operation, one circuit is in an operating state and the other circuit is in a waiting state. Furthermore, at the time of a maintenance inspection, there is commonly adopted a manner being such that the one circuit in an operating state is switched over to the other circuit in a waiting state so that the circuit in an operating state, which is subject to the maintenance inspection, is stopped (to be made into a waiting state), and the maintenance inspection is performed.
FIG. 8 illustrates a slurry-transporting facility 100 which is an example of conventional slurry-transporting facilities equipped with high-pressure pump-equipment circuits. This slurry-transporting facility 100 is equipped with two pump-equipment circuits (pump-equipment circuit X, pump-equipment circuit Y)
Here, a high-pressure pump (a transport pump) which is a constituent of each of the pump-equipment circuits is often disposed in the vicinity of the ground surface from viewpoints of convenience of maintenance inspections and the heaviness of the pump, and a reservoir tank as a transport destination for heated and pressurized ore slurry is often arranged to be kept at some distance from the ground surface. Therefore, for example, in the case of two high-pressure pump-equipment circuits, an inlet of the reservoir tank is kept distant and furthermore the foregoing expensive material is used for a pipe arrangement in the circuit, and therefore, an outlet of an ON/OFF valve 104 and an outlet of an ON/OFF valve 106 in FIG. 8 are joined to each other in the vicinity of the pumps disposed around the ground surface, and then, united into one uniting pipe arrangement 108 and connected to the reservoir tank. Furthermore, by opening and closing the ON/OFF valve 107 arranged near an outlet of the uniting pipe arrangement 108, the supply of ore slurry to the reservoir tank is controlled.
In this conventional slurry-transporting facility illustrated in FIG. 8, for example, in the case of an operation status in which a transport pump 101 of the pump-equipment circuit X is in an operating state, whereas a transport pump 102 of the pump-equipment circuit Y is in a waiting state, as a process of switchover between pumps to perform a maintenance inspection for the transport pump 101, in other words, a process of switchover from the pump-equipment circuit X to the pump-equipment circuit Y, the following operations are conducted.
That is, first, as Step S1, an ON/OFF valve 103 and the ON/OFF valve 104 in a pipe arrangement in the pump-equipment circuit X which is to be changed to a waiting state and the ON/OFF valve 107 in the uniting pipe arrangement 108 are switched from an open state to a closed state, and the transport pump 101 which is subject to a maintenance inspection is stopped. In other words, this operation allows the transportation of slurry to the reservoir tank as a transport destination to be completely stopped. Next, as Step S2, an ON/OFF valve 105 and the ON/OFF valve 106 in a pipe arrangement in the pump-equipment circuit Y which is to be changed into an operating state are switched from a closed state to an open state, and then, the transport pump 102 is operated. Next, as Step S3, when a pressure value detected by a pressure gauge 109 configured to measure a pressure in the uniting pipe arrangement 108 is larger than a pressure value detected by a pressure gauge 110 provided in the reservoir tank as a transport destination, the ON/OFF valve 107 provided in the uniting pipe arrangement 108 is switched from a closed state to an open state so that slurry transported via the pump-equipment circuit Y is made to flow into the reservoir tank.
In the conventional slurry-transporting facility, if the foregoing switchover control between the pump-equipment circuits, particularly, the pressure condition for the pressure gauge 109 and the pressure gauge 110 which is to be determined in Step S3 is not kept, contents flow backward from the reservoir tank and the pressure in the reservoir tank is decreased, and, as a result, operations are adversely affected.
However, during from the start of Step S1 to the end of Step S2, both the transport pump 101 and the transport pump 102 are in a stopped state, and, during from the start of Step S1 to the end of Step S3 (during the state of the ON/OFF valve 107 is switched from a closed state to an open state), the supply (hereinafter, also referred to as “feeding”) of slurry to the reservoir tank is stopped.
Although there are some variations, the feeding is stopped for at least 1 to 2 minutes, usually for not less than 5 minutes. When the feeding of slurry is stopped, for example, in the case where the reservoir tank as a transport destination for the slurry is not so large, an interlock operates to protect a transport pump, and the transport pump itself is stopped. Furthermore, also in an autoclave configured to apply leaching processing to transported slurry, the addition of sulfuric acid to the autoclave needs to be stopped to protect a lining of the autoclave. In this case, downstream steps are also affected, and therefore, it takes at least a few hours or half a day to regain an original normal state, and accordingly, operation efficiency considerably decreases. Furthermore, when approximately 2 hours elapse after the supply of slurry is stopped, the autoclave needs to be cooled once from a viewpoint of facility protection, and therefore, it is necessary to stop the supply for approximately one day or longer. As a result, a shutdown operation (cooling) and a startup operation (heating) for the autoclave need to be performed additionally.
For example, Patent Document 2 discloses a technique for stably transporting a high-viscosity slurry at high pressure. However, this technique is to properly operate a high-pressure pump provided at some position in one circuit, and Patent Document 2 does not disclose a technique which makes it possible that, at the time of switchover between two high-pressure-pump-equipment circuits provided to be switched over for maintenance inspections and the like, a continuous operation (slurry supply) is performed with preventing the occurrence of a state in which the feeding is stopped. Furthermore, the slurry to be subject to the transport is wastewater sludge, human waste, purifying-tank sludge, crushed resin, and the like, and accordingly the technique does not offer high versatility for, for example, slurries which have different properties, such as viscosity, temperature, and pH (for example, the foregoing ore slurry of nickel oxide ore), and is not effectively applied thereto.